Wide bandwidth envelope trackers

ABSTRACT

High bandwidth envelope trackers are provided herein. In certain embodiments, an envelope tracking system for a power amplifier includes a switching regulator that operates in combination with a high bandwidth amplifier to generate a power amplifier supply voltage for the power amplifier based on an envelope of a radio frequency (RF) signal amplified by the power amplifier. The high bandwidth amplifier includes an output that generates an output current for adjusting the power amplifier supply voltage, a first input that receives a reference signal, and a second input that receives an envelope signal indicating the envelope of the RF signal. The second input has lower input impedance than the first input to provide a rapid transient response and high envelope tracking bandwidth.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. § 119 ofU.S. Provisional Patent Application No. 62/522,951, filed Jun. 21, 2017and titled “WIDE BANDWIDTH ENVELOPE TRACKERS,” which is hereinincorporated by reference in its entirety.

BACKGROUND Field

Embodiments of the invention relate to electronic systems, and inparticular, to power amplifiers for radio frequency (RF) electronics.

Description of the Related Technology

Power amplifiers are used in RF communication systems to amplify RFsignals for transmission via antennas. It is important to manage thepower of RF signal transmissions to prolong battery life and/or providea suitable transmit power level.

Examples of RF communication systems with one or more power amplifiersinclude, but are not limited to, mobile phones, tablets, base stations,network access points, customer-premises equipment (CPE), laptops, andwearable electronics. For example, in wireless devices that communicateusing a cellular standard, a wireless local area network (WLAN)standard, and/or any other suitable communication standard, a poweramplifier can be used for RF signal amplification. An RF signal can havea frequency in the range of about 30 kHz to 300 GHz, such as in therange of about 450 MHz to about 6 GHz for certain communicationsstandards.

SUMMARY

In certain embodiments, the present disclosure relates to a mobiledevice. The mobile device includes a transceiver configured to generatea radio frequency transmit signal, a front end circuit including a poweramplifier configured to amplify the radio frequency transmit signal, anda power management circuit including an envelope tracker configured togenerate a power amplifier supply voltage of the power amplifier. Theenvelope tracker includes a differential amplifier including an outputconfigured to provide an output current operable to adjust a voltagelevel of the power amplifier supply voltage, a first input configured toreceive a reference voltage, and a second input configured to receive anenvelope signal that changes in relation to an envelope of the radiofrequency transmit signal. The second input has an input impedance thanis lower than an input impedance of the first input.

In some embodiment embodiments, the differential amplifier includes anamplification circuit biased by a control signal, and an input currentreduction circuit configured to adjust the control signal via feedbackso as to reduce an input current to the second input. According to anumber of embodiments, the input current reduction circuit includes afirst voltage divider electrically connected between the output and thesecond input and operable to control the second input. In accordancewith various embodiments, the input current reduction circuit includes asecond voltage divider connected between the output and the second inputand operable to provide a second divided voltage, and a controlamplifier configured to generate the control signal based on a voltagedifference between the first divided voltage and the second dividedvoltage.

In several embodiments, the input impedance of the second input is atleast ten times lower than the input impedance of the first input.

In a number of embodiments, the input impedance of the second input isless than the input impedance of the first input at least over afrequency range of the envelope signal.

In various embodiments, the first input connects to a transistor gate,and the second input connects to at least one of a transistor source ora transistor drain.

In some embodiments, the first input connects to a differential inputbuffer and the second input does not connect to any differential inputbuffer.

In several embodiments, the envelope tracker further includes aswitching regulator configured to generate a regulated voltage, and acombiner configured to combine the regulated voltage and the outputcurrent to generate the power amplifier supply voltage. According to anumber of embodiments, the envelope tracker further includes a currentsource connected to the second input and a tracking circuit connected tothe combiner and configured to control a current of the current source.In accordance with various embodiments, the differential amplifier isfurther configured to generate a sense signal that tracks the outputcurrent, and the switching regulator configured to generate theregulated voltage based on the sense signal.

In certain embodiments, the present disclosure relates to an envelopetracking system. The envelope tracking system includes a power amplifierconfigured to amplify a radio frequency signal, and an envelope trackerconfigured to generate a power amplifier supply voltage of the poweramplifier. The envelope tracker includes a differential amplifierincluding an output configured to provide an output current operable toadjust a voltage level of the power amplifier supply voltage, a firstinput configured to receive a reference voltage, and a second inputconfigured to receive an envelope signal that changes in relation to anenvelope of the radio frequency signal. The second input has an inputimpedance that is lower than an input impedance of the first input.

In various embodiments, the differential amplifier includes anamplification circuit biased by a control signal, and an input currentreduction circuit configured to adjust the control signal via feedbackso as to reduce an input current to the second input. According to anumber of embodiments, the input current reduction circuit includes apair of voltage dividers electrically connected in parallel between theoutput and the second input, and the input current reduction circuit isconfigured to generate the control signal based on a voltage differencebetween the pair of voltage dividers.

In several embodiments, the input impedance of the second input is atleast ten times lower than the input impedance of the first input.

In a number of embodiments, the first input connects to a transistorgate, and the second input connects to at least one of a transistorsource or a transistor drain.

In some embodiments, the envelope tracker further includes a switchingregulator configured to generate a regulated voltage, and a combinerconfigured to combine the regulated voltage and the output current togenerate the power amplifier supply voltage.

In certain embodiments, the present disclosure relates to a method ofenvelope tracking. The method includes amplifying a radio frequencysignal using a power amplifier, and generating a power amplifier supplyvoltage of the power amplifier using an envelope tracker that includes adifferential amplifier having a first input of a first input impedanceand a second input of a second input impedance less than the first inputimpedance. Generating the power amplifier supply voltage includesreceiving a reference voltage at the first input, receiving an envelopesignal that changes in relation to an envelope of the radio frequencysignal at the second input, and adjusting a voltage level of the poweramplifier supply voltage using an output current of the differentialamplifier.

In several embodiments, the method further includes generating a controlsignal based on providing feedback from an output of the differentialamplifier to the second input of the differential amplifier, andreducing an input current to the second input by biasing thedifferential amplifier with the control signal.

In a number of embodiments, generating the power amplifier supplyvoltage further includes generating a regulated voltage using aswitching regulator of the envelope tracker, and combining the regulatedvoltage and the output current using a combiner. In accordance with someembodiments, the method further includes generating a sense signal thattracks the output current using the differential amplifier, andcontrolling the switching regulator using the sense signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of one embodiment of a communicationsystem for transmitting radio frequency (RF) signals.

FIG. 2A is a schematic diagram of one embodiment of an envelope trackingsystem for a power amplifier.

FIG. 2B is a schematic diagram of another embodiment of an envelopetracking system for a power amplifier.

FIG. 2C is a schematic diagram of another embodiment of an envelopetracking system for a power amplifier.

FIG. 3 is a schematic diagram of another embodiment of an envelopetracking system for a power amplifier.

FIG. 4 is a schematic diagram of one embodiment of a high bandwidthamplifier for an envelope tracking system.

FIG. 5 is a schematic diagram of one embodiment of a differentialamplification circuit for the high bandwidth amplifier of FIG. 4.

FIG. 6A is one example of a graph of voltage for versus time for anenvelope tracking system.

FIG. 6B is another example of a graph of voltage for versus time for anenvelope tracking system.

FIG. 7 is another example of a graph of voltage for versus time for anenvelope tracking system.

FIG. 8 is another example of a graph of voltage for versus time for anenvelope tracking system.

FIG. 9A is a graph showing a first example of power amplifier supplyvoltage versus time.

FIG. 9B is a graph showing a second example of power amplifier supplyvoltage versus time.

FIG. 10 is a schematic diagram of another embodiment of a communicationsystem.

FIG. 11A is a schematic diagram of one embodiment of a packaged module.

FIG. 11B is a schematic diagram of a cross-section of the packagedmodule of FIG. 11A taken along the lines 11B-11B.

FIG. 12 is a schematic diagram of one embodiment of a phone board.

FIG. 13 is a schematic diagram of one embodiment of a mobile device.

DETAILED DESCRIPTION OF EMBODIMENTS

The following detailed description of certain embodiments presentsvarious descriptions of specific embodiments. However, the innovationsdescribed herein can be embodied in a multitude of different ways, forexample, as defined and covered by the claims. In this description,reference is made to the drawings where like reference numerals canindicate identical or functionally similar elements. It will beunderstood that elements illustrated in the figures are not necessarilydrawn to scale. Moreover, it will be understood that certain embodimentscan include more elements than illustrated in a drawing and/or a subsetof the elements illustrated in a drawing. Further, some embodiments canincorporate any suitable combination of features from two or moredrawings.

Envelope tracking is a technique that can be used to increase poweradded efficiency (PAE) of a power amplifier by efficiently controlling avoltage level of a power amplifier supply voltage in relation to anenvelope of a radio frequency (RF) signal amplified by the poweramplifier. Thus, when the envelope of the RF signal increases, thevoltage supplied to the power amplifier can be increased. Likewise, whenthe envelope of the RF signal decreases, the voltage supplied to thepower amplifier can be decreased to reduce power consumption.

High bandwidth envelope trackers are provided herein. In certainembodiments, an envelope tracking system for a power amplifier includesa DC-to-DC converter that operates in combination with a high bandwidthamplifier to generate a power amplifier supply voltage for the poweramplifier based on an envelope of an RF signal amplified by the poweramplifier. The high bandwidth amplifier includes an output thatgenerates an output current for adjusting the power amplifier supplyvoltage, a first input that receives a reference signal, and a secondinput that receives an envelope signal indicating the envelope of the RFsignal. The second input has lower input impedance than the first inputto provide a rapid transient response and high envelope trackingbandwidth.

For example, the second input can source or sink a relatively largecurrent to quickly charge or discharge internal capacitances of the highbandwidth amplifier to provide a fast transient response. The secondinput can have lower input impedance than the first input at least overa frequency range of the envelope signal. In contrast, when an envelopetracker's amplifier includes a pair of inputs with high input impedance,a resistor-capacitor (RC) time constant associated with charging ordischarging capacitances can be relatively large. Thus, such envelopetrackers can operate with a delay that degrades the envelope trackingsystem's bandwidth.

In certain configurations, the high bandwidth amplifier includes aninput current reduction circuit for dynamically adjusting a controlsignal of the high bandwidth amplifier to reduce an amount of inputcurrent flowing into or out of the second input. For example, in certainimplementations the input current reduction circuit controls a voltagelevel of a control voltage of the high bandwidth amplifier'samplification circuitry to a bias point associated with low inputcurrent and fast envelope tracking.

Accordingly, an input current reduction circuit can dynamically bias thehigh bandwidth amplifier to provide wide envelope tracking bandwidth,for instance, 180 MHz or more of modulation bandwidth.

In certain configurations, the second input of the high bandwidthamplifier that receives the envelope signal can be of much lower inputimpedance the first input that receives the reference voltage. In oneexample, the second input can connect to FET drain and/or sourceregions, while the first input can connect to a gate region of muchhigher impedance. In another example, an input impedance of the secondinput is at least ten times lower than an input impedance of the firstinput.

By providing the envelope signal to a low impedance input of anamplifier, a relatively large current can be sourced or sunk as neededto quickly charge or discharge the amplifier's internal capacitances. Incontrast, when an envelope tracker's amplifier includes a pair of inputswith high input impedance, an RC time constant associated with chargingand discharging capacitances can be relatively large.

In certain implementations, the high bandwidth amplifier also generatesa sense signal, such as a sense current, that tracks the output current.The sense signal can be used in a wide variety of ways to enhance theperformance of the envelope tracking system. For example, the DC-to-DCcurrent can use the sense signal in part to generate a regulatedvoltage, which is adjusted by the output current from the high bandwidthamplifier to generate the power amplifier supply voltage.

FIG. 1 is a schematic diagram of one embodiment of a communicationsystem 50 for transmitting RF signals. The communication system 50includes a battery 1, an envelope tracker 2, a power amplifier 3, adirectional coupler 4, a duplexing and switching circuit 5, an antenna6, a baseband processor 7, a signal delay circuit 8, a digitalpre-distortion (DPD) circuit 9, an I/Q modulator 10, an observationreceiver 11, an intermodulation detection circuit 12, an envelope delaycircuit 21, a coordinate rotation digital computation (CORDIC) circuit22, a shaping circuit 23, a digital-to-analog converter 24, and areconstruction filter 25.

The communication system 50 of FIG. 1 illustrates one example of an RFsystem that can include an envelope tracking system implemented inaccordance with one or more features of the present disclosure. However,the teachings herein are applicable to RF systems implemented in a widevariety of ways.

The baseband processor 7 operates to generate an in-phase (I) signal anda quadrature-phase (Q) signal, which correspond to signal components ofa sinusoidal wave or signal of a desired amplitude, frequency, andphase. For example, the I signal and the Q signal provide an equivalentrepresentation of the sinusoidal wave. In certain implementations, the Iand Q signals are outputted in a digital format. The baseband processor7 can be any suitable processor for processing baseband signals. Forinstance, the baseband processor 7 can include a digital signalprocessor, a microprocessor, a programmable core, or any combinationthereof.

The signal delay circuit 8 provides adjustable delay to the I and Qsignals to aid in controlling relative alignment between the envelopesignal provided to the envelope tracker 2 and the RF signal RF_(IN)provide to the power amplifier 3. The amount of delay provided by thesignal delay circuit 8 is controlled based on amount of intermodulationin adjacent bands detected by the intermodulation detection circuit 12.

The DPD circuit 9 operates to provide digital shaping to the delayed Iand Q signals from the signal delay circuit 8 to generate digitallypre-distorted I and Q signals. In the illustrated embodiment, thedigital shaping provided by the DPD circuit 9 is controlled based onamount of intermodulation detected by the intermodulation detectioncircuit 12. The DPD circuit 9 serves to reduce a distortion of the poweramplifier 3 and/or to increase the efficiency of the power amplifier 3.

The I/Q modulator 10 receives the digitally pre-distorted I and Qsignals, which are processed to generate the RF signal RF_(IN). Forexample, the I/Q modulator 10 can include DACs configured to convert thedigitally pre-distorted I and Q signals into an analog format, mixersfor upconverting the analog I and Q signals to radio frequency, and asignal combiner for combining the upconverted I and Q signals into theRF signal RF_(IN). In certain implementations, the I/Q modulator 10 caninclude one or more filters configured to filter frequency content ofsignals processed therein.

The envelope delay circuit 21 delays the I and Q signals from thebaseband processor 7. Additionally, the CORDIC circuit 22 processes thedelayed I and Q signals to generate a digital envelope signalcorresponding to a digital representation of an envelope of the RFsignal RF_(IN). Although FIG. 1 illustrates an implementation using theCORDIC circuit 22, an envelope signal can be obtained in other ways.

The shaping circuit 23 operates to shape the digital envelope signal toenhance the performance of the communication system 50. In certainimplementations, the shaping circuit 23 includes a shaping table thatmaps each level of the digital envelope signal to a corresponding shapedenvelope signal level. Envelope shaping can aid in controllinglinearity, distortion, and/or efficiency of the power amplifier 3.

In the illustrated embodiment, the shaped envelope signal is a digitalsignal that is converted by the DAC 24 to an analog envelope signal.Additionally, the analog envelope signal is filtered by thereconstruction filter 25 to generate an envelope signal suitable for useby the envelope tracker 2. In certain implementations, thereconstruction filter 25 includes a low pass filter.

With continuing reference to FIG. 1, the envelope tracker 2 receives theenvelope signal from the reconstruction filter 25 and a battery voltageV_(BATT) from the battery 1, and uses the envelope signal to generate apower amplifier supply voltage V_(CC) _(_) _(PA) for the power amplifier3 that changes in relation to the envelope of the RF signal RF_(IN). Thepower amplifier 3 receives the RF signal RF_(IN) from the I/Q modulator10, and provides an amplified RF signal RF_(OUT) to the antenna 6through the duplexing and switching circuit 5, in this example.

The directional coupler 4 is positioned between the output of the poweramplifier 3 and the input of the duplexing and switching circuit 5,thereby allowing a measurement of output power of the power amplifier 3that does not include insertion loss of the duplexing and switchingcircuit 5. The sensed output signal from the directional coupler 4 isprovided to the observation receiver 11, which can include mixers forproviding down conversion to generate downconverted I and Q signals, andDACs for generating I and Q observation signals from the downconverted Iand Q signals.

The intermodulation detection circuit 12 determines an intermodulationproduct between the I and Q observation signals and the I and Q signalsfrom the baseband processor 7. Additionally, the intermodulationdetection circuit 12 controls the DPD provided by the DPD circuit 9and/or a delay of the signal delay circuit 8 to control relativealignment between the envelope signal and the RF signal RF_(1N). Inanother embodiment, the intermodulation detection circuit 12additionally or alternatively controls a delay of the signal delaycircuit 21.

By including a feedback path from the output of the power amplifier 3and baseband, the I and Q signals can be dynamically adjusted tooptimize the operation of the communication system 50. For example,configuring the communication system 50 in this manner can aid inproviding power control, compensating for transmitter impairments,and/or in performing DPD.

Although illustrated as a single stage, the power amplifier 3 caninclude multiple stages. Furthermore, the teachings herein areapplicable to communication systems including multiple power amplifiers.

FIGS. 2A-3 are schematic diagram of various embodiments of envelopetracking systems for a power amplifier. The envelope tracking systems ofFIGS. 2A-3 illustrate various embodiments of envelope tracking systemsthat provide fast envelope tracking. However, the teachings herein areapplicable to envelope trackers implemented in a wide variety of ways.Accordingly, other implementations are possible.

FIG. 2A is a schematic diagram of one embodiment of an envelope trackingsystem 60 for a power amplifier 51. The envelope tracking system 60includes a DC-to-DC converter 52, a high bandwidth amplifier 53, an ACcombiner 54, and a feedback circuit 55. The high bandwidth amplifier 53is implemented in accordance with one or more features of the presentdisclosure.

In the illustrated embodiment, the DC-to-DC converter 52 operates togenerate a regulated voltage V_(REG) based on a DC-to-DC referencevoltage V_(REF′) and a sense signal SENSE from the high bandwidthamplifier 53. The DC-to-DC converter 52 can be implemented in a widevariety of ways including, but not limited to, using a buck converter, aboost converter, or a buck-boost converter. The DC-to-DC converter 52 isalso referred to herein as a switching regulator.

The sense signal SENSE serves to track changes in an envelope signalENV. For example, the sense signal SENSE can change in relation to anoutput current of the high bandwidth amplifier 53. However, otherimplementations are possible.

The high bandwidth amplifier 53 includes a first input that receives areference voltage V_(REF) and a second input that receives the envelopesignal ENV. In the illustrated embodiment, the first input is anon-inverting input and the second input is an inverting input. However,other implementations are possible.

The high bandwidth amplifier 53 further includes an output that iselectrically connected to the second input via the feedback circuit 55.The feedback circuit 55 can be implemented in a wide variety of ways. Inone example, the feedback circuit 55 includes at least one of a resistoror a capacitor, for instance, a parallel combination of a resistor and acapacitor.

The AC combiner 54 operates to combine the output of the DC-to-DCconverter 52 and the output of the high bandwidth amplifier 53 togenerate a power amplifier supply voltage V_(CC) _(_) _(PA) for thepower amplifier 51. The power amplifier 51 amplifies an RF input signalRF_(IN) to generate an RF output signal RF_(OUT). The envelope trackingsystem 60 receives the envelope signal ENV, which changes in relation toan envelope of the RF input signal RF_(IN).

With continuing reference to FIG. 2A, the inverting input of the highbandwidth amplifier 53 that receives the envelope signal ENV isimplemented with lower input impedance relative to the non-invertinginput that receives the reference voltage V_(REF). For example, withrespect to internal amplification circuitry of the high bandwidthamplifier 53, the inverting input can connect to transistor drain and/orsource regions, while the non-inverting input can connect to atransistor gate region of much higher impedance.

By providing the envelope signal to a low impedance input, a relativelylarge current can be sourced or sunk as needed to quickly charge ordischarge internal capacitances of the amplifier 53. In contrast, whenan envelope tracker's amplifier includes a pair of inputs with highinput impedance, an RC time constant associated with charging anddischarging capacitances can be relatively large.

FIG. 2B is a schematic diagram of another embodiment of an envelopetracking system 70 for a power amplifier. The envelope tracking system70 includes a DC-to-DC converter 52, a high bandwidth amplifier 53, anAC combiner 54, a feedback circuit 55, a current source 56, and a DCtracking circuit 57.

The envelope tracking system 70 of FIG. 2B is similar to the envelopetracking system 60 of FIG. 2A, except that the envelope tracking system70 further includes the current source 56 and the DC tracking circuit57.

The current source 56 is electrically connected to the second input ofthe high bandwidth amplifier 53, and provides a current that iscontrolled by the DC tracking circuit 57. The DC tracking circuit 57monitors the AC combiner 54 (for instance, one or more currents and/orvoltages), and adjusts the current of the current source 56 to maintainsuitable DC biasing levels.

FIG. 2C is a schematic diagram of another embodiment of an envelopetracking system 80 for a power amplifier. The envelope tracking system80 includes a DC-to-DC converter 52, a high bandwidth amplifier 53, anAC combiner 54, a feedback circuit 55, and a differential amplifier 59.

The envelope tracking system 80 of FIG. 2C is similar to the envelopetracking system 60 of FIG. 2A, except that the envelope tracking system80 further includes the differential envelope amplifier 59. As shown inFIG. 2C, the differential envelope amplifier 59 generates the envelopesignal ENV based on a differential envelope signal including anon-inverted signal component ENV_p and an inverted signal componentENV_n. Including the differential envelope amplifier 59 can enhancesystem performance by providing superior noise rejection.

FIG. 3 is a schematic diagram of another embodiment of an envelopetracking system 150 for a power amplifier 51. The envelope trackingsystem 150 includes a DC-to-DC converter 102, a high bandwidth amplifier103, an AC combiner 104, a DC tracking circuit 106, a first currentsource 107, a second current source 108, a DAC 109, a first feedbackresistor 111, a second feedback resistor 112, and an input resistor 113.

The DC-to-DC converter 102 includes a switcher 121, an inductor 122, anoutput capacitor 123, a hysteretic current comparator 124, and a voltageadder 125. The DAC 109 receives a digital reference signal REF_DAC,which controls a voltage of the DC-to-DC reference voltage V_(REF′)generated by the DAC 109. In certain implementations, the digitalreference signal REF_DAC is received over an interface, for instance, aserial bus such as the Mobile Industry Peripheral Interface (MIPI) RadioFrequency Front-End Control Interface (RFFE) bus 651 of FIG. 10.

The hysteretic current comparator 124 processes a sense currentI_(SENSE) from the high bandwidth amplifier 103 to generate a correctionvoltage V_(COR). The voltage adder 125 adds the correction voltageV_(COR) and the DC-to-DC reference voltage V_(REF′) to generate acontrol voltage of the switcher 121. The switcher 121 receives a batteryvoltage V_(BATT) and a ground voltage, and controls a current flowingthrough the inductor 122 over time to control a voltage level of aregulated voltage V_(REG) at the output of the DC-to-DC converter 102.

Including the hysteretic current comparator 124 aids in controlling thevoltage level of the regulated voltage V_(REG) based on the sensecurrent I_(SENSE) so as to reduce an average output current of the highbandwidth amplifier 103. Since the DC-to-DC converter 102 can have ahigher efficiency than that of the high bandwidth amplifier 103,reducing the average output current of the high bandwidth amplifier 103can improve the overall efficiency of the envelope tracking system 150.

The AC combiner 104 includes an inductor 131 and an AC couplingcapacitor 132. As shown in FIG. 3, the inductor 131 is connected betweenthe regulated voltage V_(REG) and the power amplifier supply voltageV_(CC) _(_) _(PA), and the AC coupling capacitor 132 is connectedbetween the power amplifier supply voltage V_(CC) _(_) _(PA) and theoutput of the high bandwidth amplifier 103.

In the illustrated embodiment, the first feedback resistor 111 isconnected between the output and inverting input of the high bandwidthamplifier 103. Additionally, the second feedback resistor 112 isconnected between the power amplifier supply voltage V_(CC) _(_) _(PA)and the inverting input of the high bandwidth amplifier 103. Althoughone example of feedback for a high bandwidth amplifier is shown, a widevariety of implementations of feedback can be used.

With continuing reference to FIG. 3, the envelope signal ENV is providedto the inverting input of the high bandwidth amplifier 103 via the inputresistor 113. Additionally, the non-inverting input of the highbandwidth amplifier 103 receives the reference voltage V_(REF). The highbandwidth amplifier 103 generates an output current, which is providedto the power amplifier supply voltage V_(CC) _(_) _(PA) via thecapacitor 132 to provide voltage level adjustment to the power amplifiersupply voltage V_(CC) _(_) _(PA). In the illustrated embodiment, thehigh bandwidth amplifier 103 also generates the sense current I_(SENSE),which changes in relation to the amplifier's output current.

In the illustrated embodiment, the inverting input of the high bandwidthamplifier 103 that receives the envelope signal ENV is implemented withlower input impedance relative to the non-inverting input that receivesthe reference voltage V_(REF). Thus, the high bandwidth amplifier 103provides fast envelope tracking.

The first current source 107 and the second current source 108 areelectrically connected in parallel with one another between theinverting input of the high bandwidth amplifier 103 and ground. The DCtracking circuit 106 controls a current of the first current source 107based on a voltage across the capacitor 132. In particular, the DCtracking circuit 106 controls the current to maintain the voltage acrossthe capacitor 132 relatively constant, thereby helping to maintainsufficient voltage headroom and suitable DC biasing at the output of thehigh bandwidth amplifier 103.

In the illustrated embodiment, the DC tracking circuit 106 includes afirst amplifier 141, a second amplifier 142, a voltage reference source143, an output amplifier 144, a first resistor 151, a second resistor152, a third resistor 153, and a capacitor 154. The first amplifier 141receives the voltage across the capacitor 132, and the second amplifier142 receives the voltage from the voltage reference source 143. In thisexample, the first and second amplifiers 141, 141 are transconductanceamplifiers, and the output amplifier 144 is a voltage amplifier.

The DC tracking circuit 106 serves to control the current of the firstcurrent source 107 such that the voltage across the capacitor 132 isabout equal to that of the voltage reference source 143. For example,when the voltages are equal, the inverting input of the output amplifier144 is about equal to the reference voltage V_(REF″). However, when thevoltages are unequal, the first amplifier 141 generates a voltage changeto the inverting input of the output amplifier 144, which in turnadjusts the current of the first current source 107 to provide DCtracking.

FIG. 4 is a schematic diagram of one embodiment of a high bandwidthamplifier 200 for an envelope tracking system. The high bandwidthamplifier 200 includes a differential amplification circuit 181 and aninput current reduction circuit 182.

The high bandwidth amplifier 200 of FIG. 4 illustrates one embodiment ofthe high bandwidth amplifier of FIGS. 2A-3. Although one example of ahigh bandwidth amplifier is shown, the high bandwidth amplifier of FIGS.2A-3 can be implemented in other ways.

The differential amplification circuit 181 serves to provideamplification between a first input IN_(p) and a second input IN_(n),which receive a reference signal and an envelope signal, respectively.The differential amplification circuit 181 further generates an outputcurrent on the output OUT. The output current is used to adjust avoltage level of a power amplifier supply voltage.

With continuing reference to FIG. 4, the differential amplificationcircuit 181 includes a first input V_(p) with relatively high inputimpedance and a second input V_(n) with relatively low input impedance.The differential amplification circuit 181 has been annotated to show afirst input current I_(B1) into the first input V_(p) of about 0 mA, anda second input current I_(B2) into the second input V_(n).

Implementing the second input V_(n) of the differential amplificationcircuit 181 with low input impedance provides a rapid transient responseand high envelope tracking bandwidth, since the envelope signal isprovided to the second input V_(n). For example, the second input V_(n)can source or sink a relatively large current to quickly charge ordischarge internal capacitances of the differential amplificationcircuit 181 to provide a fast transient response. In certainconfigurations, an input impedance into the second input V_(n) is atleast a factor of ten lower than an input impedance into the first inputV_(p).

As shown in FIG. 4, the input current reduction circuit 182 controls acontrol voltage V_(CTRL) of the differential amplification circuit 181.As a voltage level of the control voltage V_(CTRL) changes, biasing ofthe differential amplification circuit 181 varies, which results in theinput current I_(B2) into the second input V_(n) changing.

The input current reduction circuit 182 serves to dynamically adjust thecontrol voltage V_(CTRL) to reduce the input current I_(B2). Biasing thedifferential amplification circuit 181 at a bias level in which theinput current I_(B2) is relatively small further enhances the speed ofamplification circuitry. For example, the differential amplificationcircuit 181 can rapidly respond to an increase or decrease in theenvelope signal when biased in this manner.

Accordingly, including the input current reduction circuit 182 aids inproviding wide envelope tracking bandwidth, for instance, 180 MHz ormore of modulation bandwidth.

In the illustrated embodiment, the input current reduction circuit 182includes a first voltage divider 189 including a first resistor 191 ofresistance R1 and a second resistor 192 of resistance R2 connected inseries between the second input IN_(n) and the output OUT. The secondinput IN_(n) is connected to the differential amplification circuit 181via the first resistor 191, and thus the first voltage divider 189serves to detect changes in the second input current I_(B2). The inputcurrent reduction circuit 182 further includes a second voltage divider190 including a third resistor 193 of resistance N*R1 and a fourthresistor 194 of resistance N*R2 connected in series between the secondinput IN_(n) and the output OUT. The input current reduction circuit 182further includes a control amplifier 195 configured to generate thecontrol voltage V_(CTRL) based on a voltage difference between a firstdivided voltage generated by the first voltage divider 189 and a seconddivided voltage generated by the second voltage divider 190.

In certain implementations, N is greater than one, for example, by afactor of 5 or more, thereby implementing the second voltage divider 190with greater resistivity than the first voltage divider 189.

FIG. 5 is a schematic diagram of one embodiment of a differentialamplification circuit 300 for the high bandwidth amplifier 200 of FIG.4. Although one example of a suitable differential amplification circuitis shown, a high bandwidth amplifier can include amplification circuitryimplemented in a wide variety of ways.

As shown in FIG. 5, the differential amplification circuit 300 includesp-type field-effect transistors (PFETs) 201, 211, 212, 225, 226, 227,231, 236, 238, 243, 244, 253, and 254. The differential amplificationcircuit 300 further includes n-type field-effect transistors (NFETs)202, 213, 214, 221, 222, 223, 232, 235, 237, 241, 242, 251, and 252. Thedifferential amplification circuit 300 further includes first to fourthbias current sources 261-264, respectively, a class AB current source265, and a class AB bias circuit 266. Additionally, the differentialamplification circuit 300 includes a buffer 204. As shown in FIG. 5, thebuffer 204, the PFET 211, the PFET 212, the NFET 213, and the NFET 214operate as a first input circuit 203. The buffer circuit 204 includes apositive input of high impedance (for instance, a transistor gate), anda negative input of high impedance (for instance, a transistor gate).

The differential amplification circuit 300 includes a first input V_(p)and a second input V_(n), which are of different input impedance. Inparticular, the first input V_(p) is connected to the buffer 204, whichcan include a transistor gate and/or other high input impedanceelements. In contrast, the second input V_(n) connects to sources ofPFET 201 and NFET 202 and drains of NFET 222 and PFET 226 at a lowimpedance node N_(LOW).

By implementing the second input V_(n) that receives the envelope signalwith low impedance, the second input V_(n) can source or sink arelatively large current to quickly charge or discharge internalcapacitances of the differential amplification circuit 300 in responseto changes in the envelope signal. Thus, high bandwidth is provided.

The differential amplification circuit 300 generates an output currentat an output OUT, and a differential sense current at outputs I_(SENSE)_(_) _(p) and I_(SENSE) _(_) _(n). Additionally, the differentialamplification circuit 300 operates with a class AB bias circuit 266 toprovide a push-pull output stage for enhanced bandwidth. Thedifferential amplification circuit 300 is also implemented to operatewith relatively low swing of internal nodes, thus further providing fasttransient response by reducing the amount of charging and dischargingneeded to respond to a change in the envelope signal.

With continuing reference to FIG. 5, the differential amplificationcircuit 300 receives a reference voltage V_(REF), which can be the sameor a different reference voltage from the reference voltage V_(REF) ofFIGS. 2A-3.

The differential amplification circuit 300 further receives the controlvoltage V_(CTRL), which can be controlled by an input current reductioncircuit, such as the input current reduction circuit 182 of FIG. 4.

FIG. 6A is one example of a graph 501 of voltage for versus time for anenvelope tracking system. The graph 501 corresponds to a three tonesimulation including a tone of 1 megahertz (MHz), 84 MHz, and 85 MHz.Although the graph 501 shows results for one example of an envelopetracking system implemented in accordance with the teachings herein,other results are possible. For example, the performance characteristicsof an envelope tracking system can depend on implementation,application, simulation or lab setup, operating conditions, and/or awide variety of other factors.

FIG. 6B is another example of a graph 502 of voltage for versus time foran envelope tracking system. The graph 502 corresponds to a three tonesimulation including a tone of 1 MHz, 84 MHz, and 85 MHz. The graph 501and the graph 502 are simulated for different load conditions of theenvelope tracking system.

FIG. 7 is another example of a graph 503 of voltage for versus time foran envelope tracking system. The graph 503 corresponds to a two tonesimulation including a tone of 180 MHz and a tone of 182 MHz.

FIG. 8 is another example of a graph of voltage for versus time for anenvelope tracking system. The graph of FIG. 8 corresponds to a portionof the graph 503 of FIG. 7 between 540 ns and 660 ns.

Although various example of performance characteristics for an envelopetracking system have been shown, other results are possible. Forexample, the performance characteristics of an envelope tracking systemcan depend on implementation, application, simulation or lab setup,operating conditions, and/or a wide variety of other factors.

FIG. 9A is a graph 617 showing a first example of power amplifier supplyvoltage versus time. The graph 617 illustrates the voltage of an RFsignal 611, the RF signal's envelope 612, and a power amplifier supplyvoltage 613 versus time. The graph 617 corresponds to one example ofwaveforms for an implementation in which the power amplifier supplyvoltage 613 is substantially fixed.

It can be important that the power amplifier supply voltage 613 of apower amplifier has a voltage greater than that of the RF signal 611.For example, powering a power amplifier using a power amplifier supplyvoltage that has a magnitude less than that of the RF signal can clipthe RF signal, thereby creating signal distortion and/or other problems.Thus, it can be important the power amplifier supply voltage 613 begreater than that of the envelope 612. However, it can be desirable toreduce a difference in voltage between the power amplifier supplyvoltage 613 and the envelope 612 of the RF signal 611, as the areabetween the power amplifier supply voltage 613 and the envelope 612 canrepresent lost energy, which can reduce battery life and increase heatgenerated in a wireless device.

FIG. 9B is a graph 618 showing a second example of power amplifiersupply voltage versus time. The graph 618 illustrates the voltage of anRF signal 611, the RF signal's envelope 612, and a power amplifiersupply voltage 614 versus time. The graph 618 corresponds to one exampleof waveforms for an implementation in which the power amplifier supplyvoltage 614 is generated by envelope tracking.

In contrast to the power amplifier supply voltage 613 of FIG. 9A, thepower amplifier supply voltage 614 of FIG. 9B changes in relation to theenvelope 612 of the RF signal 611. The area between the power amplifiersupply voltage 614 and the envelope 612 in FIG. 9B is less than the areabetween the power amplifier supply voltage 613 and the envelope 612 inFIG. 9A, and thus the graph 618 of FIG. 9B can be associated with anenvelope tracking system having greater energy efficiency.

FIG. 10 is a schematic diagram of another embodiment of a communicationsystem 660. The communication system 660 further includes a transceiver641, a power amplifier module 642, a transmit filter module 643, areceive filter module 644, a low noise amplifier (LNA) module 645, anantenna switch module 646, a coupler module 647, a sensor module 648, apower management module 649, an antenna 650, and a MIPI RFFE bus 651.

As shown in FIG. 10, various components of the communication system 660are interconnected by the MIPI RFFE bus 651. Additionally, thetransceiver 641 includes a master device of the MIPI RFFE bus 651, andeach of the RF components includes a slave device of the MIPI RFFE bus651. The master device of the transceiver 641 sends control commandsover the MIPI RFFE bus 651 to configure the communication system 660during initialization and/or while operational.

The power amplifier module 642 can include one or more power amplifiers.As shown in FIG. 10, the power amplifier module 642 receives one or morepower amplifier supply voltages from the power management module 649.The power management module 649 can include an envelope tracker thatgenerates at least one power amplifier supply voltage, and that isimplemented in accordance with the teachings herein.

Although FIG. 10 illustrates one example of a communication systemincluding a power management module and a power amplifier module, theteachings herein are applicable to communication systems implemented ina wide variety of ways.

FIG. 11A is a schematic diagram of one embodiment of a packaged module700. FIG. 11B is a schematic diagram of a cross-section of the packagedmodule 700 of FIG. 11A taken along the lines 11B-11B. The packagedmodule 700 illustrates an example of a module that can include circuitryimplemented in accordance with one or more features of the presentdisclosure.

The packaged module 700 includes a first die 701, a second die 702,surface mount components 703, wirebonds 708, a package substrate 720,and encapsulation structure 740. The package substrate 720 includes pads706 formed from conductors disposed therein. Additionally, the dies 701,702 include pads 704, and the wirebonds 708 have been used to connectthe pads 704 of the dies 701, 702 to the pads 706 of the packagesubstrate 720.

In certain implementations, the dies 701, 702 are manufactured usingdifferent processing technologies. In one example, the first die 701 ismanufactured using a compound semiconductor process, and the second die702 is manufactured using a silicon process. Although an example withtwo dies is shown, a packaged module can include more or fewer dies.

The packaging substrate 720 can be configured to receive a plurality ofcomponents such as the dies 701, 702 and the surface mount components703, which can include, for example, surface mount capacitors and/orinductors.

As shown in FIG. 11B, the packaged module 700 is shown to include aplurality of contact pads 732 disposed on the side of the packagedmodule 700 opposite the side used to mount the dies 701, 702.Configuring the packaged module 700 in this manner can aid in connectingthe packaged module 700 to a circuit board such as a phone board of awireless device. The example contact pads 732 can be configured toprovide RF signals, bias signals, ground, and/or supply voltage(s) tothe dies 701, 702 and/or the surface mount components 703. As shown inFIG. 11B, the electrically connections between the contact pads 732 andthe dies 701, 702 can be facilitated by connections 733 through thepackage substrate 720. The connections 733 can represent electricalpaths formed through the package substrate 720, such as connectionsassociated with vias and conductors of a multilayer laminated packagesubstrate.

In some embodiments, the packaged module 700 can also include one ormore packaging structures to, for example, provide protection and/orfacilitate handling of the packaged module 700. Such a packagingstructure can include overmold or encapsulation structure 740 formedover the packaging substrate 720 and the components and die(s) disposedthereon.

It will be understood that although the packaged module 700 is describedin the context of electrical connections based on wirebonds, one or morefeatures of the present disclosure can also be implemented in otherpackaging configurations, including, for example, flip-chipconfigurations.

FIG. 12 is a schematic diagram of one embodiment of a phone board 750.The phone board 750 includes an envelope tracking module 752 and a poweramplifier module 751 attached thereto. In certain configurations, thepower amplifier module 751 and/or the envelope tracking module 752 areimplemented using a module similar to that of the module 700 shown inFIGS. 11A-11B. As shown in FIG. 12, the envelope tracking module 752provides a power amplifier supply voltage V_(CC) _(_) _(PA) to the poweramplifier module 751. Additionally, the envelope tracking module 752controls the power amplifier supply voltage V_(CC) _(_) _(PA) to changein relation to the envelope of an RF signal amplified by the poweramplifier module 751.

Although not illustrated in FIG. 12 for clarity, the phone board 750typically includes additional components and structures.

FIG. 13 is a schematic diagram of one embodiment of a mobile device 800.The mobile device 800 includes a baseband system 801, a transceiver 802,a front end system 803, antennas 804, a power management system 805, amemory 806, a user interface 807, and a battery 808.

The mobile device 800 can be used communicate using a wide variety ofcommunications technologies, including, but not limited to, 2G, 3G, 4G(including LTE, LTE-Advanced, and LTE-Advanced Pro), 5G, WLAN (forinstance, Wi-Fi), WPAN (for instance, Bluetooth and ZigBee), WMAN (forinstance, WiMax), and/or GPS technologies.

The transceiver 802 generates RF signals for transmission and processesincoming RF signals received from the antennas 804. It will beunderstood that various functionalities associated with the transmissionand receiving of RF signals can be achieved by one or more componentsthat are collectively represented in FIG. 13 as the transceiver 802. Inone example, separate components (for instance, separate circuits ordies) can be provided for handling certain types of RF signals.

The front end system 803 aids is conditioning signals transmitted toand/or received from the antennas 804. In the illustrated embodiment,the front end system 803 includes power amplifiers (PAs) 811, low noiseamplifiers (LNAs) 812, filters 813, switches 814, and duplexers 815.However, other implementations are possible.

For example, the front end system 803 can provide a number offunctionalizes, including, but not limited to, amplifying signals fortransmission, amplifying received signals, filtering signals, switchingbetween different bands, switching between different power modes,switching between transmission and receiving modes, duplexing ofsignals, multiplexing of signals (for instance, diplexing ortriplexing), or some combination thereof.

In certain implementations, the mobile device 800 supports carrieraggregation, thereby providing flexibility to increase peak data rates.Carrier aggregation can be used for both Frequency Division Duplexing(FDD) and Time Division Duplexing (TDD), and may be used to aggregate aplurality of carriers or channels. Carrier aggregation includescontiguous aggregation, in which contiguous carriers within the sameoperating frequency band are aggregated. Carrier aggregation can also benon-contiguous, and can include carriers separated in frequency within acommon band or in different bands.

The antennas 804 can include antennas used for a wide variety of typesof communications. For example, the antennas 804 can include antennasassociated transmitting and/or receiving signals associated with a widevariety of frequencies and communications standards.

In certain implementations, the antennas 804 support MIMO communicationsand/or switched diversity communications. For example, MIMOcommunications use multiple antennas for communicating multiple datastreams over a single radio frequency channel. MIMO communicationsbenefit from higher signal to noise ratio, improved coding, and/orreduced signal interference due to spatial multiplexing differences ofthe radio environment. Switched diversity refers to communications inwhich a particular antenna is selected for operation at a particulartime. For example, a switch can be used to select a particular antennafrom a group of antennas based on a variety of factors, such as anobserved bit error rate and/or a signal strength indicator.

The mobile device 800 can operate with beamforming in certainimplementations. For example, the front end system 803 can include phaseshifters having variable phase controlled by the transceiver 802.Additionally, the phase shifters are controlled to provide beamformation and directivity for transmission and/or reception of signalsusing the antennas 804. For example, in the context of signaltransmission, the phases of the transmit signals provided to theantennas 804 are controlled such that radiated signals from the antennas804 combine using constructive and destructive interference to generatean aggregate transmit signal exhibiting beam-like qualities with moresignal strength propagating in a given direction. In the context ofsignal reception, the phases are controlled such that more signal energyis received when the signal is arriving to the antennas 804 from aparticular direction. In certain implementations, the antennas 804include one or more arrays of antenna elements to enhance beamforming.

The baseband system 801 is coupled to the user interface 807 tofacilitate processing of various user input and output (110), such asvoice and data. The baseband system 801 provides the transceiver 802with digital representations of transmit signals, which the transceiver802 processes to generate RF signals for transmission. The basebandsystem 801 also processes digital representations of received signalsprovided by the transceiver 802. As shown in FIG. 13, the basebandsystem 801 is coupled to the memory 806 of facilitate operation of themobile device 800.

The memory 806 can be used for a wide variety of purposes, such asstoring data and/or instructions to facilitate the operation of themobile device 800 and/or to provide storage of user information.

The power management system 805 provides a number of power managementfunctions of the mobile device 800. The power management system 805 caninclude an envelope tracker 860 implemented in accordance with one ormore features of the present disclosure.

As shown in FIG. 13, the power management system 805 receives a batteryvoltage form the battery 808. The battery 808 can be any suitablebattery for use in the mobile device 800, including, for example, alithium-ion battery.

CONCLUSION

Some of the embodiments described above have provided examples inconnection with mobile devices. However, the principles and advantagesof the embodiments can be used for any other systems or apparatus thathave needs for envelope tracking.

Unless the context clearly requires otherwise, throughout thedescription and the claims, the words “comprise,” “comprising,” and thelike are to be construed in an inclusive sense, as opposed to anexclusive or exhaustive sense; that is to say, in the sense of“including, but not limited to.” The word “coupled”, as generally usedherein, refers to two or more elements that may be either directlyconnected, or connected by way of one or more intermediate elements.Likewise, the word “connected”, as generally used herein, refers to twoor more elements that may be either directly connected, or connected byway of one or more intermediate elements. Additionally, the words“herein,” “above,” “below,” and words of similar import, when used inthis application, shall refer to this application as a whole and not toany particular portions of this application. Where the context permits,words in the above Detailed Description using the singular or pluralnumber may also include the plural or singular number respectively. Theword “or” in reference to a list of two or more items, that word coversall of the following interpretations of the word: any of the items inthe list, all of the items in the list, and any combination of the itemsin the list.

Moreover, conditional language used herein, such as, among others,“can,” “could,” “might,” “can,” “e.g.,” “for example,” “such as” and thelike, unless specifically stated otherwise, or otherwise understoodwithin the context as used, is generally intended to convey that certainembodiments include, while other embodiments do not include, certainfeatures, elements and/or states. Thus, such conditional language is notgenerally intended to imply that features, elements and/or states are inany way required for one or more embodiments or that one or moreembodiments necessarily include logic for deciding, with or withoutauthor input or prompting, whether these features, elements and/orstates are included or are to be performed in any particular embodiment.

The above detailed description of embodiments of the invention is notintended to be exhaustive or to limit the invention to the precise formdisclosed above. While specific embodiments of, and examples for, theinvention are described above for illustrative purposes, variousequivalent modifications are possible within the scope of the invention,as those skilled in the relevant art will recognize. For example, whileprocesses or blocks are presented in a given order, alternativeembodiments may perform routines having steps, or employ systems havingblocks, in a different order, and some processes or blocks may bedeleted, moved, added, subdivided, combined, and/or modified. Each ofthese processes or blocks may be implemented in a variety of differentways. Also, while processes or blocks are at times shown as beingperformed in series, these processes or blocks may instead be performedin parallel, or may be performed at different times.

The teachings of the invention provided herein can be applied to othersystems, not necessarily the system described above. The elements andacts of the various embodiments described above can be combined toprovide further embodiments.

While certain embodiments of the inventions have been described, theseembodiments have been presented by way of example only, and are notintended to limit the scope of the disclosure. Indeed, the novel methodsand systems described herein may be embodied in a variety of otherforms; furthermore, various omissions, substitutions and changes in theform of the methods and systems described herein may be made withoutdeparting from the spirit of the disclosure. The accompanying claims andtheir equivalents are intended to cover such forms or modifications aswould fall within the scope and spirit of the disclosure.

What is claimed is:
 1. A mobile device comprising: a transceiverconfigured to generate a radio frequency transmit signal; a front endcircuit including a power amplifier configured to amplify the radiofrequency transmit signal; and a power management circuit including anenvelope tracker configured to generate a power amplifier supply voltageof the power amplifier, the envelope tracker including a differentialamplifier including an output configured to provide an output currentoperable to adjust a voltage level of the power amplifier supplyvoltage, a first input configured to receive a reference voltage, and asecond input configured to receive an envelope signal that changes inrelation to an envelope of the radio frequency transmit signal, thesecond input having an input impedance than is lower than an inputimpedance of the first input.
 2. The mobile device of claim 1 whereinthe differential amplifier includes an amplification circuit biased by acontrol signal, and an input current reduction circuit configured toadjust the control signal via feedback so as to reduce an input currentto the second input.
 3. The mobile device of claim 2 wherein the inputcurrent reduction circuit includes a first voltage divider electricallyconnected between the output and the second input and operable tocontrol the second input.
 4. The mobile device of claim 3 wherein theinput current reduction circuit includes a second voltage dividerconnected between the output and the second input and operable toprovide a second divided voltage, and a control amplifier configured togenerate the control signal based on a voltage difference between thefirst divided voltage and the second divided voltage.
 5. The mobiledevice of claim 1 wherein the input impedance of the second input is atleast ten times lower than the input impedance of the first input. 6.The mobile device of claim 1 wherein the first input connects to atransistor gate, and the second input connects to at least one of atransistor source or a transistor drain.
 7. The mobile device of claim 1wherein the first input connects to a differential input buffer and thesecond input does not connect to any differential input buffer.
 8. Themobile device of claim 1 wherein the envelope tracker further includes aswitching regulator configured to generate a regulated voltage, and acombiner configured to combine the regulated voltage and the outputcurrent to generate the power amplifier supply voltage.
 9. The mobiledevice of claim 8 wherein the envelope tracker further includes acurrent source connected to the second input and a tracking circuitconnected to the combiner and configured to control a current of thecurrent source.
 10. The mobile device of claim 8 wherein thedifferential amplifier is further configured to generate a sense signalthat tracks the output current, the switching regulator configured togenerate the regulated voltage based on the sense signal.
 11. Anenvelope tracking system comprising: a power amplifier configured toamplify a radio frequency signal; and an envelope tracker configured togenerate a power amplifier supply voltage of the power amplifier, theenvelope tracker including a differential amplifier including an outputconfigured to provide an output current operable to adjust a voltagelevel of the power amplifier supply voltage, a first input configured toreceive a reference voltage, and a second input configured to receive anenvelope signal that changes in relation to an envelope of the radiofrequency signal, the second input having an input impedance that islower than an input impedance of the first input.
 12. The envelopetracking system of claim 11 wherein the differential amplifier includesan amplification circuit biased by a control signal, and an inputcurrent reduction circuit configured to adjust the control signal viafeedback so as to reduce an input current to the second input.
 13. Theenvelope tracking system of claim 12 wherein the input current reductioncircuit includes a pair of voltage dividers electrically connected inparallel between the output and the second input, the input currentreduction circuit configured to generate the control signal based on avoltage difference between the pair of voltage dividers.
 14. Theenvelope tracking system of claim 11 wherein the input impedance of thesecond input is at least ten times lower than the input impedance of thefirst input.
 15. The envelope tracking system of claim 11 wherein thefirst input connects to a transistor gate, and the second input connectsto at least one of a transistor source or a transistor drain.
 16. Theenvelope tracking system of claim 11 wherein the envelope trackerfurther includes a switching regulator configured to generate aregulated voltage, and a combiner configured to combine the regulatedvoltage and the output current to generate the power amplifier supplyvoltage.
 17. A method of envelope tracking, the method comprising:amplifying a radio frequency signal using a power amplifier; andgenerating a power amplifier supply voltage of the power amplifier usingan envelope tracker that includes a differential amplifier having afirst input of a first input impedance and a second input of a secondinput impedance less than the first input impedance, generating thepower amplifier supply voltage including receiving a reference voltageat the first input, receiving an envelope signal that changes inrelation to an envelope of the radio frequency signal at the secondinput, and adjusting a voltage level of the power amplifier supplyvoltage using an output current of the differential amplifier.
 18. Themethod of claim 17 further comprising generating a control signal basedon providing feedback from an output of the differential amplifier tothe second input of the differential amplifier, and reducing an inputcurrent to the second input by biasing the differential amplifier withthe control signal.
 19. The method of claim 17 wherein generating thepower amplifier supply voltage further includes generating a regulatedvoltage using a switching regulator of the envelope tracker, andcombining the regulated voltage and the output current using a combiner.20. The method of claim 19 further comprising generating a sense signalthat tracks the output current using the differential amplifier, andcontrolling the switching regulator using the sense signal.